A screw is a bolt which may comprise a drive, wherein the bolt is provided with a thread at the outside. In screws with a head, the annular area under the head serves as a stop against the part to be fixed with it.
DE 27 54 870 A1 discloses a self-tapping screw of a kind which has a good hold in chipboards. Such a screw has a self-tapping thread, wherein a constriction is formed between two thread turns. Material can migrate into the constriction or indentation, which is displaced by the thread turns, whereby the load-bearing capacity shall be increased. The thread portion is arranged between a head and an end which may have a drilling tip. A screw with the drilling tip may be used for drilling holes into metal sheets, wherein, after the drilling tip passes through, the screw is directly screwed into the borehole.
DE 10 2006 020 630 A1 discloses a screw for fixing a plastic window frame to a window reveal made of wood. The screw has a screw head and a screw tip with a cutting notch, wherein a screw thread region with a coarse thread is provided between the screw head and the screw tip.
DE 202 10 464 U1 discloses a screw, in particular for a use with wood, with a shaft which is provided with an outer thread, wherein a respective thread valley between two thread hills is formed arch-shaped. The thread valley may be formed with chamfers between the thread hills. By the thread valleys formed like this, the torque and the bendability of the screw shall be increased and a breakage shall be avoided.
CH 7671 A discloses a wood screw with a thread made of thread turns which are formed of a concave boundary line at a back side and a boundary line at a front side. These thread turns are arranged between a head and a tapered, peaked end.
EP 0 243 526 A1 discloses a hammering in screw with a head and a shaft portion provided with thread webs. An end of the hammering in screw is flattened. Between the windings of the thread web, a substantially cylindrical shaft wall area is provided, respectively. For setting the hammering in screw, machine-made prefabricated accommodation bores for the hammering in screw are provided in a blind frame and a sash frame, respectively.
From EP 0,589,398 B1 and DE 10 2010 028 344 A1, respectively, for a base made of plastic or a light metal, it is known to screw a screw with a thread into the base, wherein non-cylindrical core portions are arranged between the thread turns, which taper towards the screw head. Thereby, the material of the base made of plastic or light metal, which is displaced when screwing in, shall merely flow to a backward direction, longitudinally to each core portion, up to the core portion having the smallest diameter, to enable a reduced screwing in torque of the screw when screwing in, after a predrilling which is normally performed previously.
For inserting a screw into a base made of plastic, it is intended to predrill a hole into the plastic base, and to provide this when molding (original forming), respectively, and to subsequently screw a screw into the hole. The hole is predrilled with a diameter which is larger than the shaft diameter of the screw. Portions of a screw for a plastic base may have a shape between neighboring thread turns, which deviates from a circular cylinder geometry. At a screw from the company Reyher, referred to as RST, for screwing into a predrilled plastic base, portions of the screw between neighboring thread turns may be formed tapering away from the screw head towards the opposite screw end. Descriptively, the high thread turns of such screws for plastic bases cut into the plastic material which abuts upon the predrilling bore hole. In other words, the anchorage effect of such screws is merely based on the anchorage of the thread turns in the predrilled base, whereas the screw shaft in the predrilled bore hole is substantially exposed.
Entirely different technical principles and physical framework conditions as for bases made of plastic or light metal apply for inserting a screw into wood. When inserting a screw into wood, the forming of a predrilling is normally omitted, instead, a wood screw is typically predrilling-freely screwed into a massive wood base. The thread of a wood screw cuts its counter thread into the wood base by itself. An example for a known wood screw is the screw Assy Plus or Assy 3.0 distributed by the company Würth. A measure for the quality of the anchorage of a wood screw in a wood base is the pull-out value. It denotes the tensile force which is required for pulling the wood screw out of the wood base, therefore for overcoming the holding force of the wood screw in the wood base.
Even though the known wood screw of the type Assy Plus and Assy 3.0, respectively, shows good pull-out values, a further improved pull-out strength of a wood screw is nevertheless desirable.